T. John Trout

Advances and applications of DuPont holographic photopolymers

High performance holographic photopolymer films have been developed to record reflection and transmission holograms. Recent advances in DuPont OmniDexTM materials, imaging and processing are discussed. A mechanism for holographic recording is presented along with a description of physical properties and holographic performance. Several applications including holographic optical elements are mentioned which can utilize the unique properties of these materials.

Hologram recording in du Pont's new photopolymer materials

New families of transmission and reflection holographic photopolymer materials and their performance are described. The materials are composed of polymeric binders, monomers, initiation system, and sensitizing dyes. The physical and holographic properties may be controlled by choice of components. Photopolymerization and diffusion of monomers is the proposed mechanism for recording of refractive index modulation. The materials are sensitized from the Uv to the red with typical exposure energies form 10 to 100 mJ/cm2. Hologram recording consists of exposure, UV cure, and heat processing. Significant refractive index modulation occurs during exposure. Processed holograms are insensitive to humidity and temperature. Holographic properties and performance of various formulations are discussed, as well as applications of these materials.

Photopolymer materials for color holography

DuPont has developed full color holographic photopolymer films with high sensitivity across the visible spectrum, which can be used to produce high performance full-color volume holograms and holographic optical elements. In addition, three color mastering films have been developed to produce high quality full color master holograms. Methods and results of holographic color recording and mastering in these materials are discussed. Examples of full- color display holograms and simple holographic optical elements are presented.

Diffractive printing methods using volume holograms

A unique, proprietary, holographic printing technology has been demonstrated using DuPont OmniDexTM Holographic Recording Films which can incorporate multicolor, image individualization and anticounterfeiting functions into volume holograms. Starting with a monocolor volume hologram, we describe a process using simple existing photopatterning technologies to introduce imagewise selective colors into the monochrome hologram. In this process a latent image is recorded in our OmniDexTM Color Tuning Film. This latent image is transferred to a prerecorded hologram by lamination and developed by heating. Bright, multicolor holographic images were achieved with high resolution. The process flexibility and applications of this technology to graphic arts holography and security printing will be discussed.

Enhanced reflective liquid crystal displays using DuPont holographic recording films

Holographic reflectors with high brightness and excellent environmental stability have been produced using DuPont holographic films. The center wavelength, color bandwidth and viewing cone are defined for the optimal viewing performance. Measurement methods used to quantify holographically enhanced reflective LCD performance are presented. The test results show that holographic reflectors based on the DuPont OmniDex film experienced less than 1 percent brightness degradation under 70 degrees and 95 percent relative humidity for 200 hours, with no measurable color shift. Two examples of how this technology can be extended to enhance color LCDs are also presented.

Sequential accelerated tests: Improving the correlation of accelerated tests to module performance in the field

DuPont has been working steadily to develop accelerated backsheet tests that correlate with solar panels observations in the field. This report updates efforts in sequential testing. Single exposure tests are more commonly used and can be completed more quickly, and certain tests provide helpful predictions of certain backsheet failure modes. DuPont recommendations for single exposure tests are based on 25-year exposure levels for UV and humidity/temperature, and form a good basis for sequential test development. We recommend a sequential exposure of damp heat followed by UV then repetitions of thermal cycling and UVA. This sequence preserves 25-year exposure levels for humidity/temperature and UV, and correlates well with a large body of field observations. Measurements can be taken at intervals in the test, although the full test runs 10 months. A second, shorter sequential test based on damp heat and thermal cycling tests mechanical durability and correlates with loss of mechanical properties seen in the field. Ongoing work is directed toward shorter sequential tests that preserve good correlation to field data.

Multi-stress durability testing to better predict outdoor performance of PV modules

Photovoltaic modules in the outdoor environment are subjected to a wide range of stresses which can operate simultaneously and sequentially and can vary based on climate and installation. These stresses can include temperature, temperature variation, localized heating, humidity, moisture (rain, snow, humidity, condensation), weathering, mechanical stress, abrasion and internal electric fields. These multiple stress make prediction of service lifetime challenging. Frequently resistance to an extended single stress is improperly used to assess durability. We have used sequential and simultaneous multistress exposure of materials and modules to better predict the synergistic effects of these stresses on module performance. We have also assessed the change in component materials properties to better understand performance changes. Finally, we compare these results to inspection of modules from the field to validate the test methods proposed.

Development of backsheet tests and measurements to improve correlation of accelerated exposures to fielded modules

Matching accelerated test results to field observations is an important objective in the photovoltaic industry. We continue to develop test methods to strengthen correlations. We have previously reported good correlation of FTIR spectra between accelerated tests and field measurements. The availability of portable FTIR spectrometers has made measurement in the field convenient and reliable. Recently, nano-indentation has shown promise to correlate changes in backsheet mechanical properties. A precisely shaped stylus is pressed into a sample, load vs displacement recorded and mechanical properties of interest calculated in a nondestructive test. This test can be done on full size modules, allowing area variations in mechanical properties to be recorded. Finally, we will discuss optical profilometry. In this technique a white light interferogram of a surface is Fourier transformed to produce a three-dimensional image. Height differences from 1 nm to 5 mm can be detected over an area of a few cm. This technique can be used on minimodules, and is useful to determine crack and defect dimensions. Results will be presented correlating accelerated tests with fielded modules covering spectroscopic, mechanical, and morphological changes.

Optical properties of PV backsheets: key indicators of module performance and durability

Polymeric backsheets are an important component affecting the performance and durability of photovoltaic modules. The optical properties of the backsheet should be considered in the design and performance of a photovoltaic module and the stability and durability of optical properties have an impact on power, safety and appearance. Changes in optical properties in fielded modules and accelerated durability testing are compared. IR analysis was conducted on various backsheet materials in accelerated durability testing and compared to outdoor performance to better understand the relevant chemical changes and associated degradation mechanisms. The connection between optical properties and chemical changes is discussed.

Analysis of glass-glass modules

Glass-Glass modules are gaining popularity for bifacial application and have believed advantages over PV modules with polymeric backsheets. Frameless glass-glass modules are promoted as PID-free, resistant to solvents, fire, and load stress, and capable of higher system voltages. We have found glass-glass modules run at higher operating temperature than Glass-Flex modules, and this reduces power output. Field power output results will be presented. Impermeable glass traps chemical byproducts, and faster power degradation from corrosion has been documented. Delamination has been observed in the field with glass-glass modules. A new accelerated test replicates this delamination. PID testing results will be presented comparing Glass-Glass and Glass-Flex modules.